1. Field Of The Invention
The present invention relates generally to the field of processing of ceramic materials and more specifically to a method of rapidly sintering such materials using electromagnetic induction heating or inductive coupled plasma.
2. Background Art
It is well known in the art of processing ceramic materials to use intensive heating to densify green or pre-sintered ceramic bodies. This is known as sintering. Typical green or pre-sintered ceramic bodies normally contain voids or pores, which need to be deleted so that the material can be fully densified to achieve strength and wear resistance properties. One common process for doing that is sintering. Issued U.S. Pat. Nos. 5,656,564; 6,558,821; and 6,905,993 provide disclosures of ceramic sintering processes. Conventional sintering employs an oven that uses an electrical heating element where temperature within a relatively small insulated volume can be precisely controlled over a period of several hours. In some cases it may prove to be advantageous to perform sintering more rapidly in order to density a ceramic body while controlling grain growth which can otherwise detract from strength properties. There are also certain scenarios where rapid sintering is desirable to simply expedite the process of finalizing a ceramic body for commercial convenience. One such scenario is the creation of a ceramic dental appliance by a dentist or dental technician, particularly while a dental patient is waiting for an appliance to be constructed and installed in a dental office.
The use of high strength and wear resistant ceramic dental appliances such as crowns has become standard in the dental care industry. The material properties, as well as control of their color and textural characteristics, make ceramic appliances ideal for their use as dental restorations and appliances. Moreover, with the recent advent of intra-oral scanners, desk-top CAD/CAM fabrication digitally-controlled milling and fast firing sintering ovens, it has become possible to permit rapid fabrication of such ceramic bodies in a dental office while a patient is present. Unfortunately, even the most rapid firing sintering ovens available today, still take too much time to complete the process of fully sintering a dental appliance. Although these ovens employ state-of-the-art electrical resistance element materials such as molybdenum di-silicide, it may still require up to two hours in a best example conventional sintering oven to complete the sintering process before the appliance is ready for oral installation into a waiting patient. Such extended periods of time that need to be devoted to just the sintering step, make the overall fabrication process more expensive, more energy consumptive, more labor intensive and less convenient for dentists and patients alike. Even the most advanced conventional fast-firing sintering ovens available today, are limited to a rate of heating which is about 40° C. to 70° C. per minute and to a maximum temperature of about 1,800° C. Such limits are the result of the heat-related properties of the resistance elements which are subject to temperature-induced stress and yet are relatively expensive to replace and to the overall size and insulative quality of the furnace which takes energy to create the elevated temperature. Though relative quality and prices vary, a typical fast-firing resistance element may cost about $200.00 and must be treated with care for optimal results.
Thus, despite the technological advances in dental office appliance design and fabrication techniques, there is still a weak link in the overall process, namely the amount of time required for fully densifying the ceramic appliance by sintering.